The present invention relates to an aqueous hydrogen chloride catalyzed process for preparing polyorganosiloxane polymers with controlled low-levels of chloride and hydroxy substitution. Residual chloride and hydroxyl radicals substituted on polyorganosiloxane polymers provide reactive sites on the polymer chain that can cause polymer chain extension. This polymer chain extension results in viscosity shifts of the fluid and consequently reduced shelf life. In addition, the presence of these residual reactive sites are important to the final product physical characteristics and performance. Therefore, it is not only important that chloride and hydroxy substitution be low on polyorganosiloxane polymers, it is also important to be able to control the level of these reactive sites within specified limits. Control of residual hydroxy on the polyorganosiloxane polymers allows a balance to be maintained between the need for adequate shelf life and the need for residual reactive sites which are necessary for the performance of the polymers in many of their end uses.
Current production of polyorganosiloxane polymers, for example polydimethylsiloxane, is a multi-step process. In a typical process, a first step involves hydrolyzing dimethyldichlorosilane and subsequent condensation of the hydrolysate to form an equilibrium mixture of low molecular weight permethylcyclosiloxanes and low molecular weight dimethylsiloxane linears. This condensation product is exhaustively washed to hydrolyze and remove chloride from the polymer. The wash is necessary to provide acceptably low levels of chloride in the final product.
The low molecular weight condensation products, free from chloride, are then run through a polymerization process to create high molecular weight polydimethylsiloxane fluids. Current processes require highly elevated temperatures and accordingly special equipment to run the processes. Also, considerable variability is experienced in the residual amount of hydroxy substitution of the polydimethylsiloxane fluid produced by this process.
Therefore, it is an objective of the present invention to provide a process that can be run at lower temperatures. Lower temperatures allow the use of lower cost corrosion resistant materials, such as plastics, as materials of fabrication for the reactor components. Lower temperature also results in reduced cleavage of organic groups from the silicon atom. Therefore, the polyorganosiloxane polymers will be more uniform because of reduced functional sites and branching typically caused by organic cleavage.
A second objective is to provide a polyorganosiloxane fluid with low levels of chloride and hydroxy substitution. A third objective is to provide a process whereby the level of hydroxy substitution of the product polyorganosiloxane fluid can be controlled. A fourth objective is to provide a process whereby out of specification polyorganosiloxane fluids can be re-processed with minimal detrimental alterations of the fluid. The instant process allows polyorganosiloxane fluids to be re-processed to adjust viscosity and hydroxy levels.
Prior patents teach that strong acids can be used to catalyze the condensation reaction of low molecular weight polyorganosiloxanes. However, the processes taught in the prior patents have generally resulted in polyorganosiloxane fluids with unacceptable high residual levels of chloride and/or hydroxy substitution.
Hyde, U.S. Pat. No. 2,467,976, issued Mar. 30, 1943, describes a method for increasing the average molecular weight of a completely dehydrated polydimethylsiloxane. The process comprises adding a 36 percent aqueous hydrogen chloride (HCl) solution to a polydimethylsiloxane fluid and refluxing the mixture until an increase in viscosity of the siloxane is effected. Hyde lists numerous strong acids which were considered suitable as catalysts for the described process. The strong acids are described as, for example, hydrobromic acid, boric acid, oxalic acid, benzene sulphonic acid, sulphuric acid, and phosphoric acid. A preferred temperature range for carrying out the process is given by Hyde as about 100.degree. C. to about 250.degree. C. Hyde indicates that the siloxanes formed by his described process are not heat stable.
Hyde, U.S. Pat. No. 2,779,776, issued Jan. 29, 1957, teaches that the reaction between a siloxane and an aqueous acid is reversible and that the polymer size of the siloxane at the point of equilibrium of the reversible reaction is determined by the concentration of the acid in the aqueous phase. Hyde teaches the described process can be used to produce siloxanes with hydroxy and chlorine substituted on the ends of the siloxane chains. Broad claims are made for the use of monobasic acids with a dissociation constant of at least 0.01 at 25.degree. C. The acid can be for example, iodic, perchloric, nitric, benzene sulphonic, trichloroacetic, dichloroacetic, trifluroacetic, periodic, hydrogen chloride, hydrogen bromide, and hydrogen iodide. Typical acid concentrations, in aqueous solution, were employed in the range of about 31% to 40% by weight.
Wilcock, U.S. Pat. No. 2,491,843, issued Dec. 20, 1949, describes a process for forming polyorganosiloxane polymers in which some of the silicon atoms are substituted with a hydrogen atom. The claimed process involves contacting an aqueous concentrated HCl solution and a mixture of trimethylchlorosilane and methyldichlorosilane. The process was run at temperatures as high as room temperature. A molar ratio of one part trimethylchlorosilane to at least 5 parts methyldichlorosilane was used. The examples used a starting concentration of 35 weight percent HCl in water.
Patrode, U.S. Pat. No. 2,469,888, issued May 10, 1949, describes a process for producing polyorganosiloxanes using a strong acid as a catalyst. A liquid polymeric organosiloxane whose structural units corresponded substantially to the formula R.sub.2 SiO and an organosiloxane having the general formula R.sub.3 SiOSiR.sub.3 were reacted in the presence of concentrated sulfuric acid.
Schwenker, U.S. Pat. No. 2,758,124, issued Aug. 7, 1956, teaches a continuous process for hydrolyzing an organosilane. The process comprises simultaneously passing a mixture of an organochlorosilane and water, which may contain up to about 32 percent by weight of HCl, into a circulating system. The feed of organochlorosilane and water is continued, with partial overflow of the formed polyorganosiloxanes and acid-containing water of greater HCl concentration (than the original feed comprised). The overflow is separated into the formed polyorganosiloxanes and acid containing water. The acid containing water is recycled into the system. Schwenker teaches the acid concentration in the aqueous phase is not critical and may be varied over a range of about 25 to 36 weight percent of HCl, with little effect on the quality of the final product. Schwenker teaches the temperature can be varied from about 25.degree. C. to 80.degree. C.
The above cited art does not recognize that hydrogen chloride can be used as a catalyst for the condensation of polyorganosiloxanes, end-substituted with hydroxyl radicals and chloride atoms, to produce high molecular weight polyorganosiloxanes with low levels of chloride and hydroxy substitution. The inventors have discovered that by using triorganosilyl radicals as an endblocker to control polymer length, as opposed to acid concentration, the aqueous hydrogen chloride concentration and temperature of the process can be varied within defined ranges to control the level of chloride and hydroxy substitution on the ends of polyorganosiloxane polymers.
The low chloride and hydroxy substituted polyorganosiloxanes formed by the presently described method are useful, for example, as anti-flatulents, antifoam compounds, and as intermediates in the formulation of sealants. Experience suggests that residual hydroxy functionality on polyorganosiloxane polymers can interact with other components of these formulations to affect stability and/or activity of these formulations. Therefore, the ability to control the level of chloride and hydroxy substitution, even when present at low levels, is important in producing stable formulations and formulations with predictable chemical reactivity.